For the coming year, we plan to further define the structural features which are responsible for the platelet cofactor activity of von Willebrand protein fragments isolated after enzymatic degradation with trypsin. Mechanisms of interaction of these fragments with platelets, collagen, ristocetin and other possible interacting substances will be studied, especially with regard to elucidating the structural features that allow for the cell-to-cell interactions. Hydrodynamic properties of von Willebrand protein multimers will be studied in the ultracentrifuge in relation to specific platelet cofactor activity and also with regard to association with the Factor VIII procoagulant molecule.